THD+N vs. Frequency is shown below for 8 ohms and 4 ohms. The 20 volt at 4 ohms graph line suggests that this receiver may not do very well with 4 ohm speakers (because distortion stays relatively high all along the graph line).

THD+N vs. Power Output is shown below for 8 ohms and 4 ohms. At 8 ohms, clipping (1% THD+N) occurred at 115 watts output, while at 4 ohms, clipping occurred at 140 watts output. There is not much difference in output between the two impedance loads, so I say again that this receiver should probably be limited to use with 8 ohm speakers.